1. An oscilloscope will show you exactly what is happening in the time domain. If you see any positive peak followed by a negative peak (relative to the signal anywhere), the bandwidth of that is at least 2 times the reciprocal of the time from + to -.
2. A spectrum analyzer will display the spectrum (power in the frequency domain). The highest frequency at which there is signal power to display above the baseline noise is the highest frequency present in the signal.
3. A scanning radio will let you hear the content of the signal as you tune through the band. A single frequency will appear as a quiet carrier in AM or FM, or as a tone in single side band demodulation.